The influence of the structure of the chromatin fiber on the binding of the chemical carcinogen benzo(a)pyrene diol epoxide-1 (BPDE-1) to the genome was studied. Antibodies specific to DNA modified by BPDE-1 were elicited and used to visualize the binding sites of the carcinogen to DNA in intact cells and in polytene chromosomes by immunofluorescence and in SV40 minichromosomes by immunoreplica techniques. The results indicate that 1) in polytene chromosomes an apparent "hot spot" for antibody binding corresponds with certain transcriptionally active loci and 2) in the SV40 minichromosomes the non-nucleosomal origin of replication shows a 1.4 increase in BPDE-1 binding as compared to the rest of the chromatin. It is concluded that the packing of the cellular DNA into nucleosomal conformation does not significantly affect the binding of BPDE-1 to DNA.